The relationship between biology and sexual orientation is a subject of research. A simple and singular biological role for sexual orientation has not been conclusively demonstrated—various studies point to different, even conflicting positions—but research suggests that a combination of genetic, hormonal, and environmental (biological and social) factors determine sexual orientation. Biological factors may involve a complex interplay of genetic factors and the early uterine environment. These factors, which may be related to the development of a heterosexual, homosexual, bisexual or asexual orientation, include genes, prenatal hormones, and brain structure.

For example, Marc Breedlove has reported that the average digit ratio of lesbians is more masculine than that of straight women and which indicates that lesbians, on average, are exposed to more prenatal testosterone than are straight women. This finding joins many others that biological influences, such as prenatal testosterone and fraternal birth order, act before birth to affect the later unfolding of human sexual orientation, which is the theme for the .